The invention in this application is concerned with a method of forming thin, defect-free monocrystalline layers of semiconductor materials on insulators. The invention is particularly concerned with the formation of thin monocrystalline layers of silicon on insulators, the so-called SOI structure.
In the production of silicon on insulated structures a thin layer of polycrystalline or amorphous silicon is deposited on an insulating substrate particularly oxidized silicon or quartz. A small molten zone in the amorphous or polycrystalline silicon layer is produced by localized heating of the silicon layer which molten zone is scanned across the silicon layer while the underlying substrate is heated. As the molten layer of silicon crystallizes, the silicon layer is converted to a monocrystalline layer.
Such a procedure is well known in the art are shown for example in European Patent Application No. 0129261, Anthony, U.S. Pat. No. 4,585,493, Kline, U.S. Pat. No. 4,590,130, J. Sakurai, J. Electrochem Soc. Solid-State Science and Technology, pp. 1485-1488 (July 1986), C. L. Bleil et al, Mat. Res. Soc. Symp. Proc. Vol. 35, pp. 689-692 and Japanese Kokai 61-19116.
A problem with these prior art methods is that the recrystallization by these zone melting methods causes defects to be formed in the resultant crystalline layer. These defects are found primarily to be the presence of subgrain or low angle grain boundaries. These subgrain boundaries are believed to arise due to stress on the growing crystal. These stresses are believed to originate from such sources as chemical impurities, surface roughness, volume expansion on freezing and thermal gradients.
The Japanese Kokai, L. L. Bleil et al and J. Sakurai show examples of the use of low melting glasses such as phosphosilicate glass, borophosphosilicate glass or Corning 7059 as substrates underlying the recrystallizable silicon layer.
Prior to the instant invention applicants thought that by use of these substrates an alternate mechanism for stress relief would be provided during crystal growth so that the formation of low-angle grain boundaries would be eliminated or significantly reduced. However, the use of these substrates has not resulted in fully eliminating the formation of these boundaries.